1. Field of the Invention
The present invention relates to an image forming apparatus, such as an copying machine, printer, and facsimile machine, using a print head including a plurality of light emitting devices, and a print head used in such an apparatus.
2. Description of the Related Art
The image forming apparatus, such as a printer, using the electro-photographic system comprises an exposing device for exposing a photo-sensitive member by a light source, such as an LED, to form on the photo-sensitive member an electrostatic latent image which is to be developed.
FIG. 7 shows an LED head containing an LED array chip for an exposing device used in a conventional image forming apparatus. In an example of FIG. 7, a plurality of LEDs 1a linearly arranged in a longitudinal direction of a rectangular LED array chip 1. This array chip produces a linear output pattern in a main scanning direction. It has six hundreds of LEDs 1a arranged per inch for the image forming apparatus having a resolution of 600 dpi.
The LEDs 1a of the LED array chip 1 have different luminous strengths because of ununiformity of characteristics caused by the manufacturing tolerance. The different luminous strengths can cause ununiform image density, resulting in the poor print quality.
FIG. 8 shows an output pattern of image simulation of a conventional image forming apparatus. In an example of FIG. 8, there is a significant difference in the size of dots between the third and fourth columns of dots from the right of the drawing in each dot row extending in a horizontal direction corresponding to the respective LEDs 1a. The difference of the dot size is caused by different luminous energies of the respective LEDs 1a. This difference in the third and fourth columns of dots produces a belt pattern around the fourth column, which has a density lower than that of the periphery thereof because of the relatively low ratio of black area per unit area. This belt pattern is caused by the different image density and readily recognized by the eyes as a pattern having a strong directivity.